Light sensitive heat erasable copying system



United States Patent 3,477,850 LIGHT SENSITIVE HEAT ERASABLE COPYINGSYSTEM Elliot Barman, Braintree, Mass., assignor to. Itek Corporation,Lexington, Mass., a corporation of Delaware No Drawing. Filed July 8,1965, Ser. No. 470,579

Int. Cl. G03c 5/24 U.S. Cl. 96-48 7 Claims ABSTRACT OF THE DISCLOSURE Aphotographic process of recording an image pattern of activatingradiation comprising exposing imagewise a copy medium comprising atleast one member of the group consisting of the 6',8'-dinitro and the5-chloro-6,8'- dinitro derivatives ofl,3,3-trimethyl-spiro[(2H-1-benzopyran) -2,2-indoline] to form avisibleimage which is heat erasable.

The present invention relates to improved data storage systems and tomethods of making the same. In particular, the present invention relatesto improved data storage systems adaptable to the storage ofbinary-coded information in a novel data storage arrangement havingcolorless and colored states.

Data storage systems employing compounds capable of being convertedbetween colored and colorless states are known in the prior art, forexample, US. Patent 2,953, 454. For the most part, the systemsemploy'compounds whose stable or unexcited state at room'temperature (70F.) is the colorless form. On the application of activating energy,usually ultraviolet light, the stable colorless state is converted tothe activated colored form. Conversion from the colored to the colorless'form, for purposes of erasing stored information, is usually effectedin such prior art systems by irradiation of the colored form with light.However, a return to the stable colorless form will also occur simplywith the passage of time because of the inherent instability of thecolored form. The rate of this fading is variable depending on thecompound employed.

The present invention relates to data storage systems employingmaterials for which the form stable at room temperature is the coloredform. The colored form can be converted to a colorless or substantiallycolorless material by irradiation with visible light. The colorless formcan only be reconverted to a colored state of high optical density byheating, and has a particularly desirable low rate of spontaneousconversion to the colored state in the absence of the application ofheat. Irradiation of the colorless state with ultraviolet light onlyeifects a partial conversion to the stable colored state in the absenceof heat.

The sensitivity of the system of the present invention to irradiationwith visible light is of importance. Prior art systems, employingcolorless compounds as the room temperature stable form, are obviouslyinsensitive to visible light since the compounds transmit such light.For conversion to the colored state, prior art systems have alwaysrequired irradiation with wavelengths outside the visible, usuallyultraviolet light. The employment in the present invention of a systemsensitive to visible light "ice permits the use of modulated laser beamsfor conversion of the colored compound to its colorless counterpart. Asis known in the art, lasers permit a high concentration of energy inlight beams of extremely small area in a fashion which is as yetimpossible with ultraviolet wavelengths. Thus the data storage systemsof the present invention are adaptable to many uses, in combination withlaser beams, which must be excluded when employing systems sensitiveonly to ultraviolet light. Systems for writing with modulated laserlight are disclosed in commonly owned copending patent application Ser.No. 423,- 801, for example.

Another feature of the systems of the invention is the necessity forheating the activated colorless or substantially colorless state aboveroom temperature to return it to a stable colored form of an otpicaldensity sufficient for easy discrimination between the colored andcolorless forms. For such easy discrimination by the eye or bylight-sensitive apparatus such as photomultiplier tubes, the colored andcolorless states preferably should diifer in their absorbance by atleast about ten fold ,e.g., preferably by a diiference in opticaldensity between about 0.1 or less for the substantially colorless orcolorless state and about 1 or greater for the colored state.

Subjecting the substantially colorless activated form of the compoundsof the invention to radiation, such as ultraviolet radiation, for longperiods of time at high intensities, but at room temperature, fails toconvert the material to a colored form having an optical density greaterthan 1. That is, in the absence of heating at a temperature between F.and 200 F., the colored state induced by such irradiation is notsufl'iciently colored to discriminate it clearly from the colorlessstate. When the initial colored state has an optical density of 2 ormore before decolorization by exposure to light, irradiation of thesubstantially colorless or colorless form at room temperature iscompounds of the prior art to their colorless forms. Stated otherwise,the half-life of the activated colorless forms of the compounds used inthe present invention is considerably greater than the half-life of theactivated colored forms of compounds used in a number of data storagesystems known in the prior art. Thus, the systems of the invention aremore stable in their activated state than are those of the prior art.

The data storage systems of the present invention employ, as thecompounds which can be converted from a colored to a colorless state byirradiation. with visible light, l,3,3-trimethyl-2-[(2-oxo 3,5'dinitrocyclohexadienylidene)-ethylidene]-indoline and its S-chlorosubstituted derivative. The unsubstituted compound is prepared, asdescribed by Koelsch et al. in Journal of the American a 1 3 hemia,-S99i tY.74 38 (1952) by condensing F schers base with2-hydroXy-3,S-nitro-benzaldehyde:

The chloro compound is prepared analogously from the S-chlordsubstitutedFischers base, available commercially from CIBA.

Both of these condensation products are colored at room temperature, andcan be classified as cyanine dyes. The materials have a Wine color, andon irradiation with visible light, for example light from a photofioodlamp,

are quickly converted to a colorless state of long half-life.

Any source of visible light can be employed for changing the state ofthe colored compounds. As mentioned earlier, the systems areparticularly adaptable to use with laser beams, for example high energy(1 to watt) argon ion lasers. These lasers produce light with peakintensity at 4880A and about 5150A. However, alternative light sources,including sunlight, tungsten bulbs, xenon flash lamps, etc., can beemployed.

For conversion from the unstable colorless or substantially colorlessstate to the colored state, the data storage systems of the inventionare heated to temperatures above room temperature, namely totemperatures between about 100 C. and 200 C. Increasing the temperaturewithin this range speeds the reversal reaction, but it is preferred touse lower temperatures within this range to prolong the life of thecompounds employed.

To prepare a data storage medium according to the invention the dyes aresuitably dispersed in a solvent. The color conversion reaction occurringin the compound occurs only when the compounds are in solution, but thisterm must be understood as including both liquid and solid solutions ofthe compounds. Solutions of the dyes can be dispersed in a liquidsolvent according to the invention. However, most conveniently, thecompounds of the present invention are dissolved in a liquid solventtogether witha resinous binder, and are then cast as films or into otherconvenient shapes. Upon evaporation of the fluid solvent, the dyeremains dispersed as a solute in the resinous binder as solvent.

For example, the dyes are conveniently dissolved in a liquid solventtogether with a resinous material and are thenapplied to an opaque orpellucid (transparent or translucent) substrate. Since it is convenientto read stored information by transmission, the colored compounds of theinvention are preferably cast on a pellucid base, such as of glass orplastic. Upon irradiation with visible light of sufiicient intensity,even by flashing for periods as short as microseconds, the coloredcompounds can be converted totheir colorless state. On heating to thetemperatures earlier mentioned, an optically dense colored state isquickly regenerated.

- The presence or absence of color in such a data storage system can beemployed as a binary code for the storage of information. Alternatively,the information may be in the form of pictorial images and can be storedin a data storage system of the present invention as a pictorial image.

A better understanding of the invention and of its many adv nta e wi lbe hadv y re e ng t th l ow SP6.

cific examples given by way of illustration.

EXAMPLE 1 0.01 gm. of1,3,3-trimethyl-2-[(2'-oxo-3',5'-dinitrocyclohexadienylidene)-ethylidene]-indolinewas dissolved in 20 gms. of a 20% solution of polyvinylidene chloride(Saran F 220)in dimethylformamide. The resulting mixture was coated to awet thickness of 2 mils on a transparent polyethylene terephthalate(Mylar) film.

The resulting red-purple colored film was given successive 30 secondexposures tol'a wavelength of 520 millimicrons having an intensity ofabout 835 microwatts/cmP. Table I below shows the .absorbance or opticaldensity of the material as a function of time.

TABLE I t (seconds) Absorbance O Q -3 1.40 30 1.16 0.94 0.76 0.625 0.515210 0.375 Infinity 0.240

The reversion ofa bleached sample of this material to the colored statewas measured as a function of time at a temperature of about 74 C.(about F.) by measuring the absorbance of the sample as a function oftime at a wavelength of 540 millimicrons. Table IIbelovv shows theredevelopment of color in the sample as a function of time.

A thin film of polyvinylidene chloride containing 1,3,3-trimethyl-2[(2"-oxo 3',5 dinitrocyclohexadienylidene)-ethylidene]-5-chloroidoline on a polyethylene terephthalate substratewas prepared as in Example 1.

The conversion of the colored form of the dye in the resulting film tothe bleached form was observed on exposure of the material to aWavelength of 533 millimicrons at an intensity of about '835microwatts/cm. Thirty-second exposureswere made. The results have beentabulated below in Table III.

' TABLE III t(seconds) Absorbance 0 1.74

450 0.51 Infinity 0.28

As in Example 1, measurements were made on the rate of conversion of thebleached form to the colored form at a temperature of about 74 C.(about-465 F.) by mounting the films in a spectrophotometer holderhaving a thermostat control. The results are reported in Table IV below.

A thin fil-m of the 5-chloroindoline compound of Example 2 was dispersedin polyvinylidene chloride on a polyethylene terephthalate carrier hadan initial absorbance of 1.60 measured at 540 millimicrons. The film wasrendered substantially colorless (absorbance=0.055) by exposure to ahigh intensity source of visible light (Sylvania Sun Gun).

The colorless film was then exposed to ultraviolet light at a wavelengthof 366 millimicrons while maintained at room temperature (about 70 F.).The absorbance of the sample after 20 minutes of such exposureasymptotically approached a maximum value of about 0.450.

However, on heating for 15 minutes at 70 C. (159 F.) the samplesabsorbance reached a value of 1.75.

EXAMPLE 4 For the purpose of comparing the half-lives of the activatedstates of a typical prior art compound and a typical compound accordingto the present invention, solid solutions of (A)1,3,3-trimethylindoline-5',7'-dichloro-6'- nitrobenzopyrylspiran (taughtin US. Patent 3,100,778 as convertible from a stable colorless state toan activated colored compound) and (B) of the 1,3,3-trimethyl-2[(2- oxo3'5 dinitrocyclohexadienylidene) ethylidene] 5- chloroindoline of thepresent invention, were prepared in polyvinylidene chloride in the formof a film on a pellucid carrier.

Sample (A) was exposed to ultraviolet light for a pro longed perioduntil it had darkened to maximum absorbance. The absorbance of thesample was then measured at regular intervals at the absorption peakwavelength of 555 millimicrons with the same kept at 25 C. From theobserved rate of decrease of absorbance, a half-life for the activatedcolored state of 18.5 hours at 25 C. was calculated on the basis offirst order kinetics.

Sample (B), which was colored when prepared, was bleached by exposure toa high intensity source of visible light. With the samplethermostatically kept at 25 C., the rate of increase in the absorbanceof the sample was measured by periodic observation at 540 millimicrons,the wavelength of peak absorption. A first-order half-life of 2.26 dayswas calculated for the activated uncolored state from the measurements.

Although the various films described in the specific examples have beenmade using polyvinylidene chloride as the solvent in the preparation ofsolid solutions, because the films using this substance are particularlyeasy to work with, other liquid and solid solvents including dimethylformamide, methylene chloride, pyridine, polystyrene polymers,polycarbonate polymers (Lexan), butadiene-styrene copoly-mers, partiallyhydrolyzed polyvinyl acetate, etc., can be used according to theinvention in the preparation of data storage systems.

What is claimed is:

1. A process of recording an image pattern of activating radiationcomprising exposing imagewise a copy medium comprising a solution of atleast one of 1,3.3-trimethyl-2-[(2 oxo 3,5'dinitrocyclohexadienylidene)- ethylidene]-indoline of the generalstructure:

H3O on,

N03 [ore-on: II 0 N0:

or its S-chloro substituted derivative, which exposure bleaches exposedportions of the copy medium.

2. A process as in claim 1 wherein the bleached image is erased byheating.

3. A process as in claim 2 wherein the heating is from a temperature ofabout 100 F. to a temperature of about 200 F.

4. A process as in claim 1 wherein the bleached image is stored at roomtemperature.

5. A process of recording an image pattern of activating radiationcomprising exposing to an image pattern of visible light a copy mediumcomprising a solution of at least one of1,3,3-t1imethyl-2-[(2-oxo-3,5'-dinitrocyclohexadienylidene)-ethylidene]-indoline or its S-chloro substituted derivative which forms a visibleimage which is heat erasable.

6. A process as in claim 5 comprising the additional step of heating thecopy medium which erases the visible image previously formed by exposureto visible light.

7. A process as in claim 5 wherein the photosensitive compound is in asolid solution and wherein the imaged copy medium is stored at atemperature below about 100 F.

References Cited UNITED STATES PATENTS 3,134,674 5/1964 Brown. 3,242,1223/ 1966 Cheng. 3,102,027 8/1963 Sprague et al. 96--89 OTHER REFERENCESDay, I. H.: Thermochromism, Chem. Rev., vol. 63, 1963, pp. 65-70.

Koelsch, C. F., et al.: Some Thermochromic Spirans, J. Am. Chem. Soc.,vol. 74, 6288-89, 1952.

Berman, E.: Photochromic Spiropyrans, J. Phys. Chem., vol. 66, No. 11,Nov. 1962, p. 2275.

Heller, C. A. et al.: Photochromism, J. Phys. Chem. 65, 1908 (1961).

NORMAN G. TORCHIN, Primary Examiner R. E. FICHTER, Assistant ExaminerUS. Cl. X.R. 96-89,

